1. Technical Field
The present invention relates to wafer holders and heater units employed in wafer “probers,” wafer testing stations in which a semiconductor wafer is set on a wafer-carrying side of the wafer holder or heater unit, and a probe card is pressed against the wafer to test the wafer's electrical characteristics.
2. Description of the Related Art
Conventionally in semiconductor inspection operations, heating processes are carried out on the substrates (wafers) that bear the semiconductor devices under test. In particular, a “burn-in” procedure for preventing post-shipment incidents of failure is carried out by ramping up the wafers to a temperature higher than the temperature level at which they are normally used, to accelerate the failure of and eliminate any potentially defective semiconductor dies (chips). In a burn-in procedure, after device circuitry has been formed on a semiconductor wafer, but before the wafer is singulated into individual chips, the electrical performance of each die is measured while the wafer is heated, and defective dies are excluded. There has been strongly felt need to shorten process time in burn-in procedures, in order to boost throughput.
Heaters are employed in such burn-in testing, both for retaining the semiconductor substrates as well as for heating the semiconductor substrates. Since conventional heaters require contacting the entire surface of the wafer back side onto a ground electrode, devices made of metal have been employed. A wafer on which circuitry has been formed is placed atop a flat metallic heater, and the electrical characteristics of the dies are measured. During the measurement, because a measuring element that is referred to as a probe card and is furnished with numerous current-conducting electrode pins is pressed onto the wafer with a force of from several dozen kgf to several hundred kgf, if the heater is thin it can become deformed, such that faulty contact between the wafer and the probe pins occurs. Consequently, with the goal of maintaining heater stiffness requiring a thick metal plate of at least 15 mm thickness, heater ramp-up/down necessitates a prolonged period, which seriously impairs improvement in throughput.
A further problem is that in burn-in procedures, current flows into the dies to measure their electrical characteristics along with the heightening of chip output power in recent years, dies emit considerable heat when their electrical characteristics are tested and, depending on the situation, a die can be destroyed by its own heat emission. This has led to demands for rapid cooling of the wafer after it is tested. Meanwhile, temperature that is as uniform as possible during the measurement operation is also called for. Given these demands, copper (Cu), with a high thermal conductivity of 403 W/mK, is used as the metal constituting the heater.
To address these issues, a wafer prober that is insusceptible to deformation and has a small heat capacity is proposed in Japanese Unexamined Pat. App. Pub. No. 2001-033484, by means of, instead of a thick metal plate, a thin metal layer formed on the front side of a ceramic baseplate that despite being thin is highly stiff and not prone to deforming. This reference maintains that the heater's high degree of stiffness eliminates occurrences of faulty contact, and that the heater's small heat capacity enables shortened ramp-up and ramp-down times. The reference also has it that aluminum alloys or stainless steels can be employed as a support platform for installation of the wafer prober.
Nevertheless, as noted in Pat. App. Pub. No. 2001-033484, if only the circumferential verge of the wafer prober were supported, the heater could warp under the pressing force of the probe card, which thus necessitated providing numerous support posts, or a similar design.
Furthermore, the continuing development of ultra large-scale integration of semiconductor processes has been accompanied by an increase in the load per unit area during wafer probing tests, which in turn has called for precision aligning of the probe card with the prober. Probers routinely repeat operations of heating the wafer to a predetermined temperature, shifting into a predetermined position during the probing test, and pressing a probe card against the wafer. In the course of the operations, high positioning precision is demanded of the prober drive system in order to move it into the predetermined position.
A problem with the prober drive system, however, is that when the wafer is heated to the predetermined temperature, i.e., a temperature on the order of 100 to 200° C., the heat is transmitted to the drive system and the various metal parts of the drive system thermally expand, on account of which the system precision is compromised. In addition, the increase in load during the probing test has led to demands for stiffness in the wafer-carrying prober itself. That is, if the prober itself deforms under the load during a probing test, the problem will be that the pins on the probe card cannot contact the wafer uniformly, which will make the test impossible, or at worst, damage the wafer. Probers are consequently made larger scale in order to minimize prober deformation, but the problem with enlarging the probers is that their weight increases, and the weight increase influences the precision of the drive system. What is more, with lager prober size has come the problem that the heating up and cooling times are extensively prolonged, lowering throughput.
In order to improve throughput, meanwhile, probers are often provided with a cooling mechanism to improve ramp-up/down speed. Nevertheless, cooling mechanisms conventionally have been air-cooling, as in Japanese Unexamined Pat. App. Pub. No. 2001-033484, for example, or else a cooling plate has been provided directly beneath the metallic heater. The problem with the former case has been that inasmuch as the mechanism is air-cooling, the cooling speed is slow. The problem with the latter case has been that, with the cooling plate being metal, because the pressure of the probe card is acts directly on the cooling plate during a probing test, the plate is liable to deform.